Conventionally, cationic polymer flocculants have been used alone for dewatering treatment of sludge; however, in these years, with increase in amount of generated sludge and deterioration of characteristics of the sludge, the sludge is not satisfactorily treated by conventional cationic polymer flocculants because they are still insufficient in capacity for treating the sludge, moisture content of dewatered sludge cake, SS recovery rate, peelability of dewatered sludge cake from filter cloth, or the like; thus there has been a demand for improving these problems.
For the purpose of improving these defects of the conventional cationic polymer flocculants, various amphoteric polymer flocculants and various dewatering methods by use thereof have been proposed.
For example, the following methods have been known: (1) a method of dewatering sludge in which a specific ionic-equivalent cationic-rich amphoteric polymer flocculant is added to an organic sludge that does not contain any inorganic sludge but is supplemented with an inorganic flocculant at a pH of 5 to 8 (Japanese Patent Publication (Kokoku) No. 5-56199), (2) a method of dewatering sludge in which an acrylate based cationic polymer flocculant and an amphoteric polymer flocculant are added in combination to organic sludge having a pH of 5 to 8 (Japanese Patent No. 2933627), (3) a dewatering method in which an inorganic flocculant is added to sludge to adjust the pH to be less than 5 and then a specific composition of anionic-rich amphoteric polymer flocculant is added (Japanese Patent Publication (Kokoku) No. 6-239), and (4) a treatment method of waste water of organic nature, in which an inorganic flocculant, an anionic polymer flocculant and a cationic-rich amphoteric polymer flocculant are successively added to the waste water (Japanese Patent Laid-Open No.6-134213).
The above described dewatering methods (1) to (4) have their respective advantages; however, these methods cannot necessarily be said to be effective against the tendency for recent waste water to become hard to be dewatered.
More specifically, since the COD value after treatment of waste water has been required to be lowered, proportion of activated sludge to be treated per the whole waste water has become higher. Namely, the sludge subjected to the dewatering treatment has become higher in content of excess sludge. Also, in case of waste water resulting from papermaking, recovery rate of fibrous contents from the waste water is increased whilst fibrous contents in the sludge are lowered, and thus several cases have occurred to which conventional polymer flocculants and dewatering methods cannot be applied.
Meanwhile, in papermaking processes, when or after a filler-containing stuff is diluted to a final concentration for feeding into a papermaking machine, a retention aid is added thereto so that the filler is prevented from flowing from the papermaking machine to whitewater, thereby improving the process yield.
As the retention aid, water-soluble high-molecular-weight cationic polymers are usually used, which include, for example, cationic acrylamide based polymers, partially hydrolyzed products thereof, Mannich modification products thereof, and the like. Recently, the following methods have been noticed: a method for improving retention in paper making process in which a cationic polymer is added to a stuff, and then bentonite is added thereto (Japanese Patent Laid-Open No.4-281095) and a method in which an acrylamide based polymer and an anionic aluminum-containing silica sol are added in combination to the stuff (Japanese Patent No. 2945761). The acrylamide based polymer as used above is produced by aqueous solution polymerization or the like with solid content of monomers being set at 10 to 40% by mass, and the polymer obtained by the aqueous solution polymerization is usually in a form of highly viscous paste. Accordingly, when used as a retention aid, the polymer paste is diluted with water so as to be used as an aqueous solution of 0.1 to 0.5% by mass.
The retention aid is essentially different in timing of addition in the papermaking process and molecular weight of useful polymers from paper strength agents that are used for the purpose of enhancing bonding strength between pulp fibers, and accordingly increasing the strength to rupture, stretch and the like. More specifically, as for the timing of addition, the paper strength agent is added after a sizing reagent such as aluminum sulfate has been added to the stuff, while the retention aid is required to be added immediately before the stuff is fed to a papermaking machine so that flocs of the pulp slurry may not be destroyed. As for the molecular weight of useful polymers, the paper strength agent has a molecular weight of hundreds of thousands, while the molecular weight of the retention aid exceeds one million, and otherwise sufficient retention improvement cannot be attained. Because of such a high molecular weight, addition amount of the retention aid ranges from 0.01 to 0. 05% by mass in relation to the pulp, which is a very small amount compared to that of the strength agent which ranges from 0.1 to 0.5% by mass. As can be seen from the above described characteristics, the retention aid is required to have characteristics close to those of the flocculants used in the waste water treatment.
The retention aid is a polymer with a molecular weight exceeding one million as described above, and thus is obtained, after polymerization, as a water-containing polymer in a form that is pasty and extremely low in solubility in water, thereby causing a problem such that an extremely long time is required to completely dilute it for use as an aqueous solution in the actual papermaking process.
Furthermore, conventional retention aids are insufficient in retention, and have significant defects on alkaline papermaking. In these years, the papermaking process is promoted to change to a closed system, and accordingly the recycled water and pulp fibers are contaminated with large amounts of water-soluble inorganic matters and others originating from the fillers so that the conventional retention aids are insufficient in retention.
The present inventors have made intensive researches for the purpose of finding out a composition, a polymer flocculant and a dewatering method, which are excellent in performance of flocculation and dewatering of various types of sludge, in particular, sludge large in proportion of waste sludge, and also finding out a retention aid excellent in solubility in water and in retention in the papermaking process.